This disclosure relates to novel aqueous ink-jet inks containing an aqueous vehicle, a pigment and an alternating polyurethane as a binder.
Polyurethanes are typically products of the reaction of a polyisocyanate and a polyol. Hard segments are typically made by inclusion of a short chain diol or diamine which when reacted with isocyanates produces urea or carbamate groups within a short distance from each other. The close proximity of urea or carbamate groups is important when intermolecular H-bonding is desired. The intermolecular H-bonding results in the formation of semi-crystalline regions within the polymer matrix. The semi-crystalline regions can be viewed as intermolecular crosslinks that give polyurethanes and polyureas their toughness and elasticity. U.S. Pat. No. 5,128,434 discloses the control of hard segment size in polyurethane formation by reacting two mole equivalents of a diisocyanate with about one mole equivalent of a low molecular weight diol.
Many aqueous dispersing agents as well as binders that are used in ink jet inks are made by the copolymerization of a diisocyanate, a soft segment diol and a diol containing an ionizing group. This is known to those skilled in the art as an “A+B+B1” copolymer, where “A” represents the diisocyanate, “B” represents one of the diols and “B1” the second diol. The resulting copolymer is considered “random” since the isocyanate groups can react with the hydroxyl groups from either “B” or “B1” in a random fashion. Under these random polymerization conditions, the distribution of “B” or “B1” in the total polymer distribution will produce individual polymers that are made up of just A+B and A+B1. Skewing of the polymer composition within the polymer distribution is well knows in the addition and condensation polymers literature and finds firm fundamental understanding in science.
Compounding the problem with random copolymerization is that a hard segment typically contains a hydrophilic ionic group. Since the ionic group resides in the hard segment, separation of the acid content (acid-number) from the hard segment and changing the amount and location of acid content are not possible.
A need exists for highly stable and higher-quality inks for ink-jet applications. Although improvements in polyurethane dispersants and binders have significantly contributed to improved ink-jet inks, the current dispersants and binders still do not provide inks with the requisite stability, print nozzle health and lifetime needed for ink-jet applications. The properties of the printed ink such as durability, fastness and optical density (OD) still require improvements. The present invention satisfies this need by providing ink compositions having a binder which is a true alternating polyurethane where the content and location of the hard segment as well as the content and location of the hydrophilic ionic segment can be controlled, independent of one another.